docs: def summary(self):
      """Summary string of mean and standard deviation.
  
      Returns:
        Summary tensor.
      """
      with tf.name_scope(self._name + '/summary'):
        mean_summary = tf.cond(
            self._count > 0, lambda: self._summary('mean', self._mean), str)
        std_summary = tf.cond(
            self._count > 1, lambda: self._summary('stddev', self._std()), str)
        return tf.summary.merge([mean_summary, std_summary])